In MS-DOS and Windows Operating Systems every drive/partition gets a letter. Here are the most common ones:
A:\ and B:\ floppy drives, A:\ is reserved as that is the way it has always been, B:\ is partially reserved for 5 1/4 floppies, however it is more often still used for 3 1/2. Disks start off at C:\, however any thing (except C:\) can be given B:\ but by default it is only given to a floppy, A:\ has to be for a floppy, and unlike B:\, this cannot be overrided.
C:\ Boot Disk where Windows is located
Here is the root of C:
How to tell drive letters
My Computer
Here the persons C:\ is shown name system as it is the boot. D:\, E:\, and F:\ must either be seperate disks or partitions. A:\ is his/her floppy, and their CD-RW drive is G:\. They have a flash drive in which is H:\, this flash drive could be a PSP, Card Reader, Thumb drive, etc. Finally Dati Mobli is a network drive, which the user set to B:\ as they have no 2nd floppy drive.
Vista
, this is to show how My Computer is different in Vista, the above pic for XP is better as it has more drives, but here goes!:
They only have a CD Drive (D:\) and their boot drive (C:\), notice how the 
shows there, this is only done for C:\ and any other drives and/or partitions wont have this, the Windows logo indicated that Windows is installed and ran off that drive, which is what makes it C:\.
Other examples
These will not embed in the page but here are some other examples if the ones above weren't enough
